- Acid-base definitions
- Organic acid-base mechanisms
- Ka and acid strength
- Ka and pKa review
- Using a pKa table
- Using pKa values to predict the position of equilibrium
- Stabilization of a conjugate base: electronegativity
- Acid strength, anion size, and bond energy
- Stabilization of a conjugate base: resonance
- Stabilization of a conjugate base: induction
- Stabilization of a conjugate base: hybridization
- Stabilization of a conjugate base: solvation
Stabilization of a conjugate base: electronegativity
How electronegativity affects the stabilization of a conjugate base (when comparing elements in the same period).
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- And what about for example HCl? Cl has lower electronegativity than F. However HCl is stronger acid than HF.(19 votes)
- The reason for this is size. Size plays a very important role if the two compounds are in the same group (column). Thus, a bigger molecule (like HCl) will be able to donate its proton more easily and be more stable than HF. Similarly, HI > HBr >HCl > HF even though Fluorine is way more electronegative. So while en does increase acidity, size plays usually plays a greater factor.
Hope that helps and good luck on your endeavours. :D(25 votes)
- When we are talking about particularly strong acids, how do we keep those acids from reacting in the first place? It seems that a bottle of HF, for example, will just have a bunch of protons floating around in solution and wouldn't be stable at all, or even be HF in aqueous form.(5 votes)
- Surprisingly, although HF is extremely corrosive, it is a weak acid (K_a =7.2×10⁻⁴).
It exists in in aqueous solution mainly as HF(aq):
HF(aq) + H₂O(l) ⇌ H₃O⁺(aq) + F⁻(aq)(3 votes)
- Are weak acids more stable than strong acids? Or does the stability depend solely on the conjugate base? Thanks(3 votes)
- I believe if we apply the analogy of conjugate bases to acids as well we can see that weak acids are indeed more stable than strong acids since they are less likely to donate a proton, hence they're more stable in the sense that they're more "comfortable" with the proton configuration that they're in - Just like a more stable conjugate base is a compound that isn't as "proton hungry" as a non-stable conjugate base which is "hungry for a proton" to stabilize itself.(1 vote)
- what do you mean when you say a Stable base?(1 vote)
- This is super late, but when an acid donates its proton, it becomes a "conjugate base." That conjugate base is just the result of it giving up its proton, causing it to have a negative charge. A more "stable" base will have a higher electronegativity, because that atom (in this case the F) will have a higher affinity for electrons, and therefore will "hog" them to itself. This causes it to be the most stable of all the "bases" described in the video.(5 votes)
- stabler the conjugate base the higher their acidic reactivity?(1 vote)
- Yes, the more stable the conjugate base (A-), the more reactive the acid will be, thereby favoring dissociation.
HA ----> H+ + A-
I'm not a fan of using the electronegativity as an example of acidity, as it has more do with the negative charge distribution among the overall size of the anion once the hydrogen dissociates.(2 votes)
- Why is an acid considered strong if it can easily donate a proton?(1 vote)
- That’s like the definition lol. If an acid is ~100% ionised in solution it is defined as a strong acid.(2 votes)
- Isnt HF a very weak acid? It has strong bonds (overlap of orbitals) between the H & F, thus not allowing a stronger pull, therefore as a whole, it wont be a very good proton donor (it will still donate protons but it is unlikely to because of the strong bond).....does this sound about right?(1 vote)
- Yeah that sounds correct.(1 vote)
- If we have a stable conjugated base, then the acid from which it was formed would be considered a stable or a strong acid? And what makes it a stable or a strong acid? Because I can't really imagine an acid being stable, because shouldn't it be somewhat unstable for it to lose hydrongens?(1 vote)
- What do you mean by stable acid? As long as the acid doesn't decompose then we would consider it stable. Depending on how reactive it is in an acid/base reaction would just call it weak or strong.
As far as judging the strength of an acid based on its conjugate base; the more stable the conjugate base the stronger the acid.
Hope that helps.(1 vote)
- I've noticed, that the fewer atoms and acid has, the stronger it often is. Is it a real tendency or just some coincidence?(0 votes)
- I don't think that is a reliable trend.
HF is not a strong acid.
HClO₄ is a strong acid.
For a list of some common acids and their pKas:
- What is the elemental effect and how can you compare or use POE with molecules for elemental, inductive, resonance, and hybridization effect?(1 vote)
- [Voiceover] If H-A is a generic acid and donates this proton, the electrons in this bond, so the electrons in magenta are left behind on A to form A minus. So A minus is the conjugate base to H-A. If A minus is stable, then H-A is more likely to donate this proton. Therefore, if you want to determine the acidity of a compound, you can look at the stability of the conjugate base. The more stable the conjugate base, the more likely the acid is to donate a proton. Therefore, the more stable the conjugate base, the stronger the acid. Let's use that concept and let's look at these four compounds down here. So we'll start with methane. The PKA for this proton on methane is approximately 48. For ammonia the PKA for this proton is about 36. And if we look at water, the PKA for this proton is about 16 and finally for H-F, this proton has a PKA of about three. We know that the lower the PKA value, the stronger the acid so as we move to the right we see a decrease in PKA values from 48 to 36 to 16 to three. Therefore, as we go to the right we see an increase in acid strength. We see an increase in acid strength so H-F is the strongest acid out of these four. And if H-F is the strongest acid out of these four, then H-F must have the most stable conjugate base. So now let's think about the conjugate bases for all four of these compounds so let me go down here and we'll get some more room. If we take this proton from methane, then these electrons are left behind on the carbons. So the carbon gets a negative one formal charge. For ammonia if we took this proton, then these electrons are left on the nitrogen so the nitrogen has a negative one formal charge. For water, if we took this proton these electrons are left on the oxygen to form the hydroxide anion as our conjugate base with a negative one formal charge on the oxygen. And finally if we took this proton, then these electrons would be left behind on the fluorine to form the fluoride anion. We already know that H-F is the strongest acid out of these four and the strongest acid must have the most stable conjugate base. So the fluoride anion must be the most stable conjugate base so as we move to the right we're increasing in the stability. We're increasing in the stability of the conjugate base and we can explain that trend by looking at the element that has the negative charge, right? This has a negative charge on carbon and this was our least stable conjugate base. Then we go to nitrogen with a negative charge. We get a little bit more stable. We go to oxygen with a negative one charge. We get a little bit more stable. And finally we get to fluorine with a negative charge and we had the most stable conjugate base. That's the same trend as electronegativity so if you look at carbon, nitrogen, oxygen, and fluorine as you move to the right on the periodic table you know you increase in electronegativity with fluorine being the most electronegative element and the most electronegative element attracts electrons the most. It likes to have electrons around it and therefore it makes sense that fluorine is the best at stabilizing a negative charge and that makes this the most stable conjugate base. And if this is the most stable conjugate base, the fluoride anion is the most stable conjugate base, that means that H-F must be the strongest acid. So when you're thinking about acid strength, think about the stabilization of the conjugate base and the different factors that can do that. Electronegativity is one of those factors to think about.